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TDA1549T

Stereo 4fs data input up-sampling filter with bitstream continuous calibration DAC (BCC-DAC1)

Philips Electronics 

Philips Semiconductors

Stereo 4fs data input up-sampling filter with

bitstream continuous calibration DAC (BCC-DAC1)

Objective specification

TDA1549T

FUNCTIONAL DESCRIPTION

General

The TDA1549T CMOS digital-to-analog bitstream

continuous calibration converter incorporates internal

digital filtering which increases the oversampling rate of 4fs

input data to 96fs, and removes the spectral data

components around 4fs, 8fs, and 12fs. A 2nd order noise

shaper operating at 96fs outputs a 5-bit data bitstream to

the DACs. The filtering required for waveform smoothing

and out-of-band noise reduction is achieved by simple 1st

order analog post-filtering (see Fig.3).

The combination of noise shaping and bitstream

continuous calibration digital-to-analog conversion

enables high performance and extremely low noise to be

achieved.

Input

The circuit accepts four times oversampled data in 18-bit

two's complement standard Japanese format with MSB

first. Left and right data channel words are time

multiplexed. The input format is illustrated in Fig.5. The bit

clock (BCK) operates at 192fs, i.e. 48 times the word select

(WS) frequency of 4fs.

Oversampling filter

The oversampling filter consists of:

• A 7th order half-band low-pass FIR filter which

increases the oversampling rate from 4 times to 8 times.

This removes the spectral components around 4fs and

12fs (see Fig.3).

• A linear interpolation section which increases the

oversampling rate to 16 times. This removes the

spectral components around 8fs.

• A sample-and-hold section which provides another

6 times oversampling to 96 times.

The zero-order hold characteristic of this sample-and-hold

section plus the 1st order analog filtering removes the

spectral components around 16fs.

Passband ripple is within 0.1 dB. Stopband attenuation is

>50 dB around multiples of the sampling frequency.

Noise Shaper

The 2nd-order digital noise shaper converts the 18-bit data

at 96fs into a 5-bit bitstream, while shifting in-band

quantization noise to frequencies well above the audio

band. For low signal levels the noise shaper output is a

1-bit bitstream. This noise shaping technique used in

combination with a special data code and bitstream DAC

enables extremely high signal-to-noise ratios to be

achieved.

Data encoder

The data encoder converts the 5-bit two's complement

output data from the noise shaper to a 32-bit thermometer

code.

In traditional unidirectional current converters, half of the

full-scale current flows to the output during small signal

reproduction. The thermal noise and substrate crosstalk

components present in this current severely restrict the

dynamic range which can be attained. In this BCC-DAC1

true low-noise performance is achieved using a special

data code and bidirectional current sources. The special

data code guarantees that only small values of current flow

to the output during small-signal passages while larger

positive or negative signals are generated using the

bidirectional current sources. For every change in the

18-bit input sample only one current source or current sink

is switched on. This intrinsically monotonic thermometer

code ensures the high differential linearity, zero crossover

distortion and superior signal-to-noise ratio associated

with bitstream conversion.

August 1994

6

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